Witnesses

refers to a record (or tuple) that provides evidence or support for an aggregate query result

SELECT x.city, MAX(y.price)
FROM Company x, Product y
WHERE x.cid = y.cid
GROUP BY x.city;

Getting the result per city, but don't know which product actually has that maximum price.

WITH CityMax AS (
  SELECT x.city, MAX(y.price) AS maxprice
  FROM Company x, Product y
  WHERE x.cid = y.cid
  GROUP BY x.city
)
SELECT DISTINCT u.city, v.pname, v.price
FROM Company u, Product v, CityMax w
WHERE u.cid = v.cid AND u.city = w.city AND v.price = w.maxprice;

cid	cname	city
1	Apple	Seattle
2	Microsoft	Seattle
3	Samsung	Seoul

pname	price	cid
iPhone	999	1
MacBook	1999	1
Surface	1499	2
Galaxy	1199	3
GalaxyTab	1999	3

city	pname	price
Seattle	MacBook	1999
Seoul	GalaxyTab	1999